At first you might think it’s a stretch for actor Chris O’Donnell and rapper LL Cool J to play special-agent buddies on the hit CBS series NCIS: Los Angeles. Even they don’t seem to be sure why they fit together so well.

“I really couldn’t explain it,” LL Cool J says with a big grin. “We had never met before the show. Then we went to the gun range for weapons training, and we just clicked. You have one guy, Chris, who’s from the Midwest and this other guy, me, who’s from the inner city of New York. But we share a common sensibility. I think that me plus him equals a little more than two, you know?”

While they come from very different places, movie star O’Donnell, 39, and rapper LL Cool J, 42, were both successful very early on. LL Cool J, whose real name is James Todd Smith, had his first hit record for Def Jam when he was 16 years old. (His stage name stands for Ladies Love Cool James.) His sexy hip-hop style and the lyrics he wrote won him Grammys and have kept him at the top of the charts for more than two decades, with hit singles such as “Going Back to Cali” and “Mama Said Knock You Out.” His friends call him Todd and describe him as a philosopher with a grand sense of adventure and joy.

O’Donnell started acting in his teens but first got wide attention when he shared the screen with Al Pacino in 1992’s Scent of a Woman. Despite his varied big-screen career, fans will always remember him as Batman’s sidekick Robin. He played the role opposite both Val Kilmer and George Clooney in hit movies in the 1990s.

“I’ve grown up a little bit,” O’Donnell reflects. “I’m almost 40 years old now. But everyone was introduced to me when I was 18 and I looked like I was 15. I’ve been around a long time. I don’t know that NCIS: Los Angeles is a complete reinvention, but I’m playing one of the guys in charge this time. Before I’d be cast as a young impressionable character. I think part of that is just being more mature.”

Despite their backgrounds, the two stars have formed a real connection on the NCIS set.

“I found out right away that neither of us has an ego that is too big to get in the way of doing what’s best for the show,” O’Donnell says. “We give each other a hard time, sometimes jokingly and sometimes seriously, but nobody gets offended. He and I are both pretty wide open. We have good communication, which is important in any relationship.” He shrugs. “We’re both kind of down-to-earth, yet we don’t have normal lives. Let’s face it—anyone who got into this business wasn’t seeking a normal life.”

LL Cool J had some good movie parts, such as in 1999’s Any Given Sunday, but NCIS: Los Angeles is his biggest hit as an actor.

“I’ve been dreaming of this since I was a little boy with a towel hanging out of my undershirt jumping off a refrigerator box pretending to be a superhero,” he says of playing an undercover special agent—who gets to use the latest high-tech equipment.

O’Donnell, who plays the more cerebral agent, agrees.

“I’ve got a gun, and I’m running around hiding behind things,” he says. “That’s what we did when we were kids down in the basement of my house. You have a couple of buddies sleep over, and, you know, you play cops and robbers. That I’m getting paid to do it now is kind of funny.”

O’Donnell was having a slow spell when NCIS: Los Angeles called. He’d made a few guest appearances on Grey’s Anatomy and starred in a Broadway play, but he admits that he was looking for a chance to heat up—or even change—his image.

“I definitely had those moments, like any actor, when you get anxious and think, ‘When am I going to work again?’ But I would feel that way even when I had every offer in the world coming to me. Then I became a father and I felt a little more of the anxiety that came with the responsibility of being a parent. I was looking for something that would allow me to stay in Los Angeles, which was best for my kids.” O’Donnell has been married since 1997 to Caroline Fentress. They have five children: Lily Anne, 10, Christopher, 9, Charles, 6, Finley, 3, and Maeve, 2.

LL Cool J hardly needed work when he joined NCIS: Los Angeles, but the series has boosted his career, too.

“My music was big, but this is the biggest thing I’ve ever done,” he says. “Housewives in Montana weren’t listening to my songs, but now they know LL Cool J because of NCIS.

“Reinvention isn’t only for celebrities or actors or musicians or athletes—reinvention is for all humanity,” LL Cool J continues. “All of us should strive for a newer and better self. Just constantly take our lives to the next level. People make a whole bunch of mistakes, and then we call it ‘experience.’ For me, having a music career for all those years kind of prepped me emotionally, psychologically, and spiritually to be able to handle something like this. You have to remember that success is a long, long road, but failure is only a step away.”

LL Cool J has been married to Simone Johnson since 1995. They have four children: Najee, 20, Italia, 19, Samaria, 14, and Nina, 9. Away from work, both he and O’Donnell are very involved parents, and that has created a bond.

“We didn’t have time to have any preconceived notions about each other,” LL Cool J says. “There just wasn’t space to wonder ‘What’s Chris gonna think of my rap background? Is he gonna be square? Is he gonna get me?’” LL Cool J leans back and beams a smile. “Then I discovered he has five kids, and I have four. Here’s the strangest part. I found out that our two fourth-graders, our 9-year-olds, are at opposing schools, and their basketball teams compete.”

As the No. 1 new series of the season, NCIS: Los Angeles is already assured a long life. After only seven weeks on the air, the show got a deal to syndicate episodes on the USA Network. Most TV series don’t get that kind of deal until they have produced close to 100 shows.

“I was aware that the show was doing well, but I didn’t allow myself to get in that mind-set of ‘I have a hit’ kind of thing,” LL Cool J says. “You don’t want to be in denial either and have some sort of pseudo humility. But success can be the greatest enemy of progress. So you have to just keep everything in the right perspective. Focus on growing as an actor.”

“Sarcasm is the key to our bonding,” O’Donnell says with a laugh. “I like to tell LL that I was watching him on Friday Night Videos when I was 15 years old. There he was onscreen with all of these women, and there I was sitting on the couch at home wishing I was out doing what he was doing!

“We’re both married, so we understand that you can’t analyze the secret to being part of a good team,” O’Donnell adds with a wink.

LL Cool J raises his eyebrows with a naughty look.

“They say that what happiness lacks in length, it makes up for in height,” he replies.